fix expected content-types for documentation tarballs

[MangoIV]

Fixes #1269

• documentation tarballs produced by cabal haddock are compressed
• their mimetype is application/gzip
• keeps the applicatoin/x-tar and applicatoin/x-gzip even though there is no tar mimetype and there's now (since 2014) a gzip mimetype, according to RFC6713
• remove the expectUncompressedTarball function as it is now dead code
• remove a pair of redundant paren and replace infix liftM with <$>

[MangoIV]

Pinging @phadej and @gbaz for review as your were participating in the original issue.

Thank you.